The Pleistocene Legacy: Beyond the Ice Front
Antony R. Orme
Among the varied landscapes of North America, a basic distinction exists between those regions that were subject to repeated Pleistocene glaciation and those that were not. The regions beyond the southern and northwestern margins of the vast ice sheets that once covered much of the continent are important because only here was sufficient land exposed above sea level to house the many terrestrial physical and biological processes at work beyond the ice front. Elsewhere, the ice sheets either calved into the deeper waters of the Atlantic and Pacific Oceans, or grounded on now-submerged continental shelves. The southern margin between glaciated and unglaciated terrain forms a broad irregular arc, convex southward, from the Atlantic continental shelf off New England to the Pacific shelf beyond Juan de Fuca Strait (fig. 3.1). The northwest margin separates glaciated terrain in southern Alaska and the Brooks Range from unglaciated Beringia, which, during eustatic lowstands, formed a broad land connection with Asia.
The ice front as defined here is not a synchronous margin but a composite of different glacial maxima that collectively represents the farthest advance of continental ice and separates glaciated terrain from those landscapes shaped by nonglacial processes. Glaciers did of course exist at higher elevations farther south. Some, such as the ice caps over the Yellowstone Plateau, the Colorado Rockies, and the Sierra Nevada, waxed and waned repeatedly during the Pleistocene, sculpting terrain and influencing regional hydrology (fig. 3.2). But beyond the main ice front, landscapes, even during glacial stages, were dominated by nonglacial processes; such glaciers as did exist were but small parts of a vast and changing landscape.
This chapter examines those components of the modern landscape that have been inherited from Pleistocene events that took place beyond the main ice front. Evidence and dating of the climate changes that generated these landscape responses are reviewed. A perspective on Pleistocene climates and vegetation is then given, emphasizing the last interglacial-glacial cycle, ≈130–10 ka (thousand years before present). Two themes are elaborated, namely, the paleohydrological implications of climate change for selected rivers and lakes, and the aeolian landscapes formed from blowing loess and sand. Other impacts of glaciation beyond the ice front, namely, eustatic and isostatic effects and some biotic anomalies, are briefly discussed. The chapter concludes with a perspective on nonglaciated North America within the broader context of late Cenozoic climate change. These events were of course played out against a backdrop of continuing tectonism, notably in the west, and of isostatic crustal adjustments, upland denudation, and downstream sediment transfers.